1. Field of the Invention
The invention relates to piston-side sealing in die-casting machines which work according to a vacuum process, in particular according to the VACURAL process, and are provided with a suction gap or suction box. (xe2x80x9cVACURALxe2x80x9d is a German trademark of Maschinenfabrik Mxc3xcller-Weingarten AG of Weingarten, Germany. Reference is made to U.S. Pat. No. 4,476,911, describing the VACURAL process.)
To increase the functional safety and the service life of the seal, sealing on a casting piston stepped in diameter is proposed.
2. Related Art
The production of light-metal parts is carried out in considerable quantities by the die-casting process. In this case, components made of aluminum and its alloys occupy a predominant position, but magnesium is becoming increasingly more attractive as a design material.
The known die-casting processes traditionally carried out are the cold-chamber process or, particularly for smaller components, the hot-chamber process. These processes have proved successful for a wide variety of parts. However, if high-grade components having minimum oxide and impurity inclusions are required, these components ensuring higher dynamic strength values and/or being subjected to a subsequent heat treatment, a vacuum die-casting process is preferably used.
In one of these processes, the metering of the melt into the filling chamber is effected via a seuction pipe located in the melting bath. This metering of the melt is brought about by the application of a vacuum, which is switched on by opening a vacuum valve after the mold is closed. In addition to this mold vacuum, an additional casting-chamber vacuum is also possible for a better and more controlled vacuum effect, as described by the applicant in DE 30 41 340 C2 as VACURAL die-casting process.
To achieve corresponding process safety and ensure an optimum vacuum effect, the entire process must as far as possible take place in a closed circuit and thus be appropriately sealed off to the outside.
In this case, a region to be sealed off is the rear side of the piston or of the piston rod which is coupled to the casting unit. The suction box which discharges the thin light-metal residues, the xe2x80x9cspanglexe2x80x9d, escaping on the rear side of the casting piston has proved successful as a seal carrier.
Thus, for example, in EP 0 462 218, a seal which seals off the casting-piston rod in the outlet region from the suction box is provided in a flanged bush fastened to the end of the suction box. A considerable disadvantage of this seal arrangement is that, in the event of the piston breaking off at the connecting thread between piston and piston rod, the sealing effect at the piston rod is retained. Normally, the cooling-water holes required for the piston cooling are located in the piston rod, so that, after the piston breaks off, the cooling water escapes from the casting-piston rod. Due to the situation which then results, the cooling water can then run back essentially only via the suction pipe into the melting bath of the holding furnace. In addition to the disturbance of the casting process and the impairment of the melt, there is then a high accident risk when casting magnesium or aluminum. Possible risks arise due to an explosive-like reaction or due to the considerable increase in volume and thus the considerable increase in pressure of the cooling water, meeting the hot melt in the suction pipe, in the resulting closed-off space. A further disadvantage of this seal arrangement is the increased seal wear if the piston and piston rod are not symmetrical to one another, i.e. if they are offset.
Sealing on a casting piston has been disclosed by U.S. Pat. No. 3,009,218; a casting piston having a uniform piston diameter over its entire length is proposed. Due to the casting process, damage, such as scouring or scores, occurs in the casting chamber, and this mechanical impairment also leads to damage to the casting piston. In the design proposed, the piston seal may then likewise be damaged and the sealing effect is not ensured in continuous operation.
The object of the invention is to develop a sealing system for a casting piston for the use of the vacuum die-casting process, which sealing system ensures a long service life and substantially reduces a risk of accidents.
According to the invention, the object is achieved in that an extended casting piston has at least one step, i.e. a smaller diameter, at its rear end, and the seal attached in the suction box is in sealing connection with the piston when the latter is being retracted. In this case, the seal is to be fitted via a frictional or positive-locking connection, and the sealing ring may have a dividing point which permits its fitting or exchange without substantial assembly measures. The casting-piston design selected, on the one hand, ensures that mechanical piston damage does not occur on the smaller seal diameter, since this diameter has no contact with the bore of the casting chamber, and, on the other hand, the casting-piston diameters are concentric due to the simple manufacturing process. Furthermore, after a piston rod breaks off, which as a rule would be effected in the region of an advanced casting position, the cooling water could escape through the annular space which is now open to the rear.
Further details and advantages of the invention follow from the description below of an exemplary embodiment.